Ink jet printing is a well known process for non-contact printing of substrates such as paper, plastic films, metal foils, and the like. In the operation of a typical ink jet printing apparatus, a stream of an ink composition is forced under pressure through a very small orifice by a drive means such as a driven piezoelectric crystal. After the stream of the ink composition is forced from the orifice, the stream separates into minute uniform size droplets. The droplets of the ink composition then receive an electrical charge. The charged droplets are deflected, causing selected droplets to impinge upon the substrate to be printed in a predetermined pattern to form a dot matrix image of the desired indicia on the substrate.
Various types of ink jet printing apparatus are known in the art. One of the more common types of ink jet printing apparatus provides a constant discharge of droplets of the ink composition in substantial excess of the amount actually required for printing, with the excess droplets being recovered and recycled. Another well known type of ink jet printing apparatus is referred to as a drop on demand printer. This type of apparatus, as the name indicates, discharges a stream of droplets of the ink composition only when required for printing. There are other types of ink jet printers known in the art, and all typically operate on the same general principles as that described above.
The various types of apparatus used for ink jet printing are relatively well developed from a mechanical standpoint. However, considerable problems are still encountered with ink jet printers. Many of the problems are related to the ink compositions heretofore suggested for use in ink jet printing. Ink compositions which are used in ink jet printing should have certain properties. Initially, in dispensing of the ink compositions during printing it is necessary that the ink compositions display a consistent breakup length, drop viscosity, and drop charge under the specific set of conditions employed during the ink jet printing process. To meet this requirement, the ink jet composition must have stable viscosity and resistive properties and should not dry out upon ageing. One of the major problems encountered with conventional prior art ink compositions is that they contain substantial amounts of either water or organic solvents which, upon standing, evaporate, causing the ink compositions to dry out and cake. This causes blocking of the orifices and considerable other problems in operation of ink jet printers. A further problem is that the loss of the volatile solvents causes the inks to increase in viscosity, which will cause substantial changes in the performance of the inks.
In addition to the unique requirements associated with ink jet printing, it is also necessary that the ink compositions used for ink jet printing have the usual desired printing properties that can be obtained with inks used in conventional printing processes. These properties include, for example, that the ink composition have satisfactory adhesion to the different types of substrates on which it may be printed. In addition, the indicia which is printed must have a high degree of resolution in order to provide the sharp images required for most printing applications. The ink composition must not rub or smudge on the substrate, and the printed indicia should likewise be permanent; that is, of archival quality.
A further problem in addition to those noted above associated with the presence of volatile solvents in the prior ink compositions used for ink jet printing is that the solvents cause the printed indicia to bleed into the substrates and result in poor resolution. To overcome this, the prior art resorted to the expedience of using coated papers which will not be wet out by the ink compositions. This is generally limited in practice because of the additional cost involved and the requirement in many commercial printing processes for "plain paper" printing. The use of large amounts of solvents in the ink composition likewise requires that the printed substrates be subjected to drying to remove the solvents. In the case of organic solvents, this has caused additional health and safety concerns.
Certain of the prior art ink compositions, in addition to containing solvents, also contain substantial amounts of wetting agents to allow the ink composition to wet out and saturate the substrate to be printed. These particular ink compositions tend to excessively wet out the substrates with solvents, and in turn cause bleeding with a resulting substantial reduction in the resolution of the printed indicia.
In typical conventional ink compositions for most other printing applications, that is, aside from ink jet printing, pigments are used as the colorants. Pigments are preferred to dyes because of the cost, consistent color quality and, more importantly, because of their outstanding permanency. However, because of the problems encountered with the prior art ink compositions used in ink jet compositions, it has not been possible to successfully use pigments in ink compositions for ink jet printing. Pigments can increase the viscosity of the ink, making it difficult to form small droplets. Dyes do not cause this problem, but require a transparent ink resin for color development.
What would be highly desirable would be an ink composition for ink jet printing which would be stable under ink jet printing conditions and which provides excellent print qualities such as high resolution pattern with a high contrast, excellent adhesion to the substrate, and also excellent archival properties.
In order to formulate an ink composition for ink jet printing, it is desirable to use a resin having a melt viscosity which is sufficiently low to facilitate passage of the ink jet composition through small orifices while at the same time, having sufficient blocking resistance and transparency properties for use in ink jet printing applications. Ink compositions must be resistant to blocking under conditions in which they are used, that is, the films formed by the ink compositions must be resistant to undesired cohesion to adjacent substrates, such as paper, ioning into contact with the printed-on indicia and to foreign particles, such as dust etc. Such conditions would include high temperature and/or humidity, whether present during application of the ink or subsequent thereto. Polyamide resins which are relatively transparent and have sufficient blocking resistance tend to have melt viscosities which are much too high to be used in an ink jet printing application. Those polyamide resins which have sufficiently low viscosity usually do not have the required blocking resistance. Those polyamide resins which do have sufficiently low viscosity and the desired blocking resistance for the ink jet printing applications are usually opaque due to their crystallinity.
Floyd, et al., U.S. Pat. No. 3,253,940 relates to polyamide resins and solutions thereof suitable for use as flexographic ink vehicles. The polyamide resins of Floyd are of relatively high viscosity, for example, about 1040 cps at 150.degree. C., due to the higher ratio of dimer to monoacids (75:25 on an equivalence basis). Based on the formulation of the polyamide resins of Floyd, the calculated average molecular weight of the resins is about 2,000 g per mole.
Drawert et al., U.S. Pat. No. 3,622,604, discloses synthetic polyamides comprising dimeric fatty acids and methods for making the same. Like Floyd et al., the polyamide resins of Drawert et al. have relatively high molten viscosities of about 1290 cps at 150.degree. C., due, again, to the higher ratio of dimer to monoacids (75:25 on an equivalence basis). Based upon the formulation of the polyamide resin in Drawert, the calculated average molecular weight will be slightly higher than that of the Floyd polyamide resin due to the high weight amines used in the Drawert polyamide resin.
Neither the resins of Floyd nor Drawert possess the required properties of low viscosity, blocking resistance and sufficient transparency, which are required in a resinous binder in order to produce ink compositions having the properties of stability and uniformity of performance under ink jet printing conditions. Such ink compositions are described in U.S. Pat. No. 4,830,671 (Frihart et al.). It is therefore an object of this invention to provide a resinous binder which satisfies the requirements of transparency, blocking resistance and low molten viscosity for producing such ink compositions.